A ScAlMgO4 substrate is a known example of a RAMO4 substrate formed of a single crystal represented by general formula RAMO4 (wherein R represents one or more trivalent elements selected from the group consisting of Sc, In, Y, and lanthanoid elements, A represents one or more trivalent elements selected from the group consisting of Fe(III), Ga, and Al, and M represents one or more divalent elements selected from the group consisting of Mg, Mn, Fe(II), Co, Cu, Zn, and Cd. For example, a ScAlMgO4 substrate is used as a growth substrate for nitride semiconductors such as GaN (see, for example, JP-A-2015-178448). FIG. 1 shows an example of a method for producing the traditional ScAlMgO4 substrate described in this related art. As shown in FIG. 1, the traditional ScAlMgO4 substrate is produced by a method that includes preparing a ScAlMgO4 bulk material, and cleaving the ScAlMgO4 bulk material.
Here, when a RAMO4 substrate is used as a seed substrate for producing a group III nitride crystal, a crystal of a group III nitride is grown on an epitaxial growth surface of the RAMO4 substrate. After being produced, the group III nitride crystal is separated from the RAMO4 substrate by taking advantage of the cleavability of the RAMO4 substrate. In this way, the method enables producing a sole group III nitride crystal. The group III nitride crystal can then be used for luminescent devices such as LEDs and lasers, and power devices such as power supplies.